Conventional seismic data processing methods are based on detecting primary reflections using a line (2D) or a grid (3D) of receivers placed on or near the surface of a zone of interest. The position of each receiver in the group is known relative to a source of seismic energy, which when triggered creates mechanical waves, which in turn activate electro-mechanical transducers that are an element of each receiver. These mechanical waves can activate one or more transducers while outbound from the source, again upon a first reflection from a subsurface element or boundary (event), and upon one or more secondary or tertiary reflections. Electro-magnetic signals generated by the transducers are recorded or “gathered” (i.e. creating a seismogram), to represent the primary reflected waves, are then position corrected or “migrated”, and later “stacked” with sibling signals recorded by the same receiver during subsequent activations of the same source in order to reduce the influence of transient noise. Using any of a number of available algorithms that accept the primary signals as data, the gathers are processed to generate images that readily reveal generally horizontal surfaces or interfaces at different depths that represent layers against which the primary waves were at different times reflected and through which those waves travel at different velocities that are characteristic of the composition of those layers.
It is well understood that information respecting primary reflections arriving at different receivers at different times from each of such multiple surfaces (subhorizontal events) can be coordinated and interpreted to accurately identify subsurface reflectors that are oriented somewhat vertically (known as subverticals), but only to within a limited range of angles (normally less than 60 degrees). For such mildly vertical reflectors this extrapolation technique using primary reflections is adequately developed, including the filtering out of more complex double reflections and ignoring as noise their influence on the resulting image. However, for steeply sloping reflectors, outside such limited range, primary reflections are insufficient to generate unambiguous images revealing their existence, position, and orientation with an acceptable level of certainty.
The development of migration procedures in recent years has permitted increased accuracy in mapping of areas with complex geology, including areas having salt domes. However, precise delineation of salt stocks, tracing of faults, and other problems connected with near-salt sediments, often still result in ambiguous solutions likely because the sub-vertical reflecting boundaries have rugose surfaces. Waves reflected only once from such boundaries, tend not to reach the surface and have been studied using “vertical seismic profiles” (VSP) according to which seismic images are created using a special migration transformation. However, the practical efficiency of such an approach is limited, because in the boreholes, such reflections can only be recorded within depth intervals deeper than the target boundary. However, some seismic waves can be reflected by sub-vertical faces of salt stocks and subsequently by sub-horizontal boundaries in adjacent sediments, permitting them to be recorded on the surface if they have enough energy to be identified against the background of other reflections. In Russia and other parts of the world, such waves have received the name “duplex”, i.e. having undergone two reflections during their propagation. Duplex waves can be formed not only under conditions of salt dome tectonics, but also in case of small-displacement faults, when the acoustic properties of the latter contrast significantly with those of host rock. This commonly happens when the subject fault is a tectonic element of a hydrocarbon trap and, therefore, the epigenetic alterations associated with the deposit result in a significant acoustic contrast across the dislocation zone. Consequently, while it is difficult to use phase-shift analysis (because of the low resolving power of known conventional seismic processing methods) duplex waves can in theory also be used to identify and trace faults with small displacements.
McMchan, G. A. (in his 1983 Article, Migration by extrapolation of time-depended boundary values: Geophys. Prosp., 31, 412-420) describes imaging vertical boundaries based on reverse time migration, however according to that method all the primary reflections were removed from the gathers, which is not realistic in processing actual seismic data.
However, in 2003 U.S. Pat. No. 7,110,323 B2, Marmalyevskyy et al teach Duplex Wave Migration (“DWM”), which allows one to build images, without preliminarily removing primary reflections, of other sources Duplex waves, as that term is used in U.S. Pat. No. 7,110,323, are the reflected portion of a source wave (whether pressure or shear) that has experienced two collisions with geologic events (e.g. a discontinuity) from which the second collision returns coherent (i.e. spatially and temporally correlated) seismic energy to the surface for observation. The collision of the source wave on the first geologic event (whether sub-horizontal or sub-vertical) generates 4 secondary waves (reflected-P, reflected-S, transmitted-P, and transmitted-S) only 2 of which (reflected-P and reflected-S) are used in the DWM method, while the 2 transmitted secondary waves are ignored or filtered out. The reflected-P and reflected-S secondary waves then each propagate to collide with a second geologic event (typically the inverse of the first collision, i.e. sub-vertical or sub-horizontal) where 4 tertiary waves arise from each of the 2 secondary waves. Again, the signal of waves transmitted through the second event are either ignored or filtered out, but the reflected-P and reflected-S pairs of tertiary waves (arising from each of those 2 secondary waves) are observed at the surface and used in the DWM method to interpret the sub-surface geology. The method of DWM reduces the calculation required to a problem of finding only one secondary source, after using conventional primary reflections and methods as input to find the first of the two secondary sources. Duplex waves are a reflected type of twice coherently deflected wave that are important but not unique to surface seismic methods. DWM is a migration procedure that allows imaging sub-vertical boundaries (such as: salt dome walls, boundaries of geological blocks, and small amplitude faults) without first removing primary reflections and where only duplex waves are used since they produce better images of steeply dipping sub-vertical boundaries typically having dip-angles in the range 60 to 90 degrees.
During modern VSP observations both converted and monotypic transmitted waves are recorded and used in many ways (e.g. define the elastic parameters of the medium, fracturing systems, absorption, signal shapes and geometry of boundaries, et cetera).
In VSP, transmitted converted waves are used for imaging without limiting the angle of inclination of the seismic boundaries. Xiao et al. (in article: Xiao, X., Zhou, M., and Schuster, G. T., 2007, Salt-flank delineation by interferometric imaging of transmitted P- to S-waves: Geophysics, 71, S1197-S1207) have shown the possibility of obtaining interferometric images of transmitted waves so as to delineate the flanks of salt domes. They used principle of stationary-phase migration by Schuster (is shown in his 2001 Article, Thyory of daylight/interferometric imaging: 63th Annual Conference and Exhibition, EAGE, Expanded Abstracts, A32) with cross-correlation between the PS and PP transmitted arrivals before phase shift.
Niheil et al, (in their 2000 Article, VSP fracture imaging with elastic reverse-time migration: 70th SEG Annual International Meeting, Expanded Abstracts) also applied interferometric principles for formation of seismic images in a wide band of angles of boundaries' inclinations. They used the RTM of VSP data to produce images of both vertical fractures and horizontal boundaries. They modified the RTM algorithm in such a way that instead of forward continuation of a compressional wave-field from a source, they used the backward continuation of down going compressional wave registered at VSP receivers.
However, transmitted waves within framework of the interferometric approach are not used in any known method of the surface seismic surveys. In difference from VSP at realization of interferometric principle for the surface seismic surveys allowing obtaining seismic images without restriction on boundaries' inclination angles, in addition to once reflected the PS and PP waves also necessary to use the duplex waves changing their modes at reflection and transmitting through those heterogeneities.
Well known techniques, such as AVO Analysis (Ostrander, 1985), use the dependence of the reflection coefficient to the angle of incidence to predict the gas saturation of rocks. However, in complex geological conditions, the obtained information from reflected waves is not enough for a confident interpretation of gas saturation.
Disadvantageously, duplex wave migration procedures that are based only on reflected waves are somewhat restricted to imaging sub-vertical boundaries with dip-angles ranging from 60-90 degrees from the horizontal axis. In VSP other types of waves are used, such as transmitted converted and non-converted waves, which allow imaging boundaries ranging from 0-90 degrees from the horizontal axis. These waves also help define elastic parameters of the medium, fracture systems, absorption, signal shapes and geometry of boundaries, etc. In surface seismic these waves are not traditionally used such that it would be valuable to find a way to use transmitted converted and non-converted waves in duplex wave migration.
The prior art in the surface seismic data processing industry has concentrated on teaching variations on methods of using reflected waves, when dealing with complex geologic media it would be advantageous to also have a method for using the information available in transmitted waves, particularly for gas saturation forecasting this would be a great advantage.